1. Field of the Invention
The present invention relates to a suspension system for a vehicle such as an automobile, and more particularly, to a hydraulic suspension system.
2. Description of the Prior Art
As described, for example, in Japanese Patent Laid-Open Publication No. Shown 63-219408 (1988), a suspension system is well known in the art as a hydraulic suspension system for a vehicle such as an automobile, which comprises actuators provided between a vehicle body and corresponding suspension arms and adapted to increase and decrease vehicle heights as the results of the supply and the discharge of working fluid into and from their working fluid chambers, respectively, accumulators connected with the working fluid chambers by passages having therein flow restrictions, working fluid supply passage means for supplying working fluid into the working fluid chambers, working fluid discharge passage means for discharging working fluid from the working fluid chambers, pressure control means such as pressure control valves which are provided in the working fluid supply passage means and the working fluid discharge passage means and which are adapted to control the supply of the working fluid into and the discharge from the working fluid chambers so as to adjust the pressures within the working fluid chambers to their desired levels.
In such a hydraulic suspension system, as the pressure within the working fluid chamber in each actuator repeatedly increases and decreases due to the bounds and rebounds of a vehicle wheel, working fluid is repeatedly supplied into and discharged from the working fluid chamber to compensate for the pressure variation within the chamber, resulting in the increased energy consumption of the working fluid. In view of this, as described, for example, in Japanese Laid-Open Publication No. Showa 62-292517 (1987), a hydraulic suspension system has been known in the art which comprises a first hydraulic system including a pressure control valve and a second hydraulic system including an accumulator and a passage having therein a flow restriction and connecting the accumulator and the working fluid chamber in the associated actuator, wherein the damping force in the first hydraulic system is set to be higher than that in the second hydraulic system when the vibration input into the actuator is higher in frequency than a predeterminate value to thereby limit the response of the pressure control valve to higher frequency vibration inputs so that the energy consumption of the working fluid may be prevented from increasing due to the bounds and rebounds of the vehicle wheel.
However, in the hydraulic suspension system described in the above-mentioned Japanese Patent Laid-Open Publication No. Showa 62-292517, since the flow resistance of the first hydraulic system is set so as to meet the above requirement by judiciously setting the inner diameter and the length of the flow line between the pressure control valve and the actuator, the freedom of setting to satisfy the requirement is rather small. That is, if the flow resistance of the first hydraulic system is set so high that the action of the pressure control valve may be effectively limited against higher frequency vibration inputs, it is too high for lower frequency vibration inputs, resulting in the aggravated response in controlling the attitude of a vehicle body. Conversely, if the flow resistance of the first hydraulic system is not set high enough, the energy consumption of the working fluid is not well reduced.